This invention relates to improved process monitoring and control using novel apparatus and methods to determine the concentration of weak acid or weak base components, or their soluble salts, in a liquid containing such acid, base or salt.
It is well known that it is useful to be able to determine the concentration of weak acid and base constituents, or their soluble salts, in liquids for the purpose of monitoring and controlling processes. Weak acids are characterized as being partially ionized in water solutions (i.e., H.sub.2 S, CO.sub.2, HCN and CH.sub.3 CO.sub.2 H), in contrast to strong acids which are fully ionized in water (i.e., H.sub.2 SO.sub.4). Weak bases (i.e., NH.sub.4 OH) can be similarly distinguished from strong bases (i.e., NaOH). Salts are produced by the reaction between acids and bases. Soluble salts are capable of mixing with liquids to form solutions. For example, the reaction between H.sub.2 S, an acid, and monoethanolamine, a base, produces monoethanolamine bisulfide, a soluble salt. Unless otherwise indicated, any reference to detecting weak acid and base components in this application should be deemed to encompass detection of the soluble salts of such weak acid or base, if any. Determination of the concentration of weak acid and base constituents, or their soluble salts, in liquids is useful in, but in no way limited to, amine system control and waste water treatment. Of particular importance is the continuous on-line measurement of the quality of industrial process streams containing weak acid and base components, or their soluble salts.
Amine system control methods based on the measurement of H.sub.2 S in rich amine are taught, for example, in U.S. Pat. Nos. 3,958,943 and 4,289,738 which are hereby incorporated by reference and made a part hereof. Typically, liquid amine is used to remove acid impurities, such as hydrogen sulfide and carbon dioxide, from gas. The amine is contacted with the gas so as to cause the impurities to be absorbed by the amine. Then the amine is regenerated by stripping acid gases out, leaving a lean amine that is suitable for recontacting with gas. Stripping is accomplished by heat input or pressure decrease. The degree to which acid gases are stripped from rich amine depends on the amount of heat used or the pressure drop. Overstripping, or removing more acid gas than necessary to regenerate amine, results in an energy penalty. On the other hand, government regulations limit the maximum amount of SO.sub.2 that may be generated when gas is combusted. Since SO.sub.2 is generated when H.sub.2 S is burned, understripping (removing too little acid gas) can result in environmental penalties or unsalable gas.
An optimum amount of acid gas should be removed from amine to avoid understripping yet minimize energy costs. Prior art teaches that amine system heat input should be based on the amount of hydrogen sulfide absorbed by the amine: the greater the amount of acid gas absorbed, the more heat input required to liberate the acid gas from the amine. Clearly, the importance of determining the concentration of weak acids in liquids is well known.
Increasingly strict environmental regulations have made it vital to prevent prohibited discharges from wastewater systems. To this end, the usefulness of monitoring waste water for NH.sub.3 content is well known. When NH.sub.3 content exceeds acceptable levels, the water can be diverted to buffer storage away from wastewater treatment systems thereby avoiding upset of the treatment process. In addition, neutralization of foul water by the addition of treating chemicals can be optimized using control based on the monitoring of weak base constituents dissolved in the water.
A review of the art reveals that limited means are presently available to achieve the desired determinations. Hydrogen sulfide detection may be attempted using several commercially available analyzers. However, no commercially available instrument is capable of essentially continuous on-line analysis of hydrogen sulfide and carbon dioxide. Further disadvantages of present analysis apparatus include: uncorrected drift, inability to distinguish interfering components from analytes and analyzer response that is adversely affected by pH, color, turbidity and temperature. As a result, commercially available hydrogen sulfide analyzers lack the degree of accuracy and repeatability necessary for continuous on-line measurement.
A method for determining the concentration of a carbonate and a sulfite in a liquid is disclosed in U.S. Pat. No. 4,663,724. A commerical device employing the teachings of the '724 patent is not available. The method involves calculating the concentrations of CO.sub.2 and SO.sub.2 in a liquid based on the concentrations of CO.sub.2 and SO.sub.2 determined for a continuously flowing sample stream, the flow rate of the sample stream and the flow rate of a carrier gas. It remains to be seen whether the '724 disclosure is practical since it relies on calculations which are very sensitive to flow measurement inaccuracies.
As previously mentioned, for amine system control it is useful to know not only H.sub.2 S content of amine, but also CO.sub.2 content. While the reasons for this are described below in detail, at this point it is sufficient to say that, in general, H.sub.2 S represents only a fraction of the total acid gas present in lean amine. Therefore, total acid gas is a better measure of lean amine quality so that H.sub.2 S and CO.sub.2 content should both be used to control regenerator heat input. Not only is instrumentation presently unavailable to make these determinations, but there is a general lack of appreciation in the art for the importance of measuring total acid gas content in amine streams used to remove impurities from gas.
The shortcomings of the art present those in processing industries with a dilemma. The usefulness and desirability of having detection apparatus and methods for continuous determination of the concentration of weak acid and base constituents dissolved in liquids is recognized, yet suitable devices and methods have not been developed to achieve this type of on-line analysis. As a result, it is customary in amine system control, for example, to obtain samples for analysis once per shift. The time lag between obtaining samples, transporting them to central laboratory facilities and performing analyses frustrates the effective utilization of analysis results in process control. By the time process operations are modified based on this sample analysis, conditions have already changed. While such time lag in obtaining analysis results impairs continuous feedback process control, it is fatal to most continuous feed-forward process controls. This is because feed-forward systems adjust downstream operating conditions in response to variations in upstream influent quality. The determination of influent quality must be on-line and continuous in such a system.
Similarly, typical waste treating control involves capturing samples for analysis in a laboratory. Water treating chemical addition rates may be based on analysis performed as infrequently as once per month. The time lag between sampling and process control using analysis results impairs effective upset prevention and causes uneconomical treatment of waste water.